A multi-threaded large scale computer system, such as a database management system (xe2x80x9cDBMSxe2x80x9d), supports a number of different users concurrently. In a multi-threaded computer system there is only one execution of the software; that is, only one process. From the one process, a user thread is created for each user. All the user threads share the same process memory space, because they are part of the same process.
A cache is a storage area operating between a processor and another, slower storage area (such as a disk). Although, other schemes may exist, typical cache memory is evenly divided into a fixed number of finitely sized cache memory blocks, called a page. The cached data includes pages which have stored therein currently executing instructions and currently referenced data. The page stored in each cache memory block is typically controlled and managed through control blocks, there being a correspondence between a control block and a cache memory block. If a user thread references an instruction or data not in memory; then a page fault occurs, which causes the relevant page to be read from disk into the cache. Such an arrangement is typical of cache memory. Problems occur when more pages need to be cached than there are available cache blocks in the cache requiring reclamation of pages.
In accordance with a particular embodiment of the invention, a public memory structure is utilized to store data that is shareable between a plurality of users in a multi-threaded computing environment. In contrast to the prior art, a cache memory area on a server is used to store public, shareable data and private, non-shareable data without using locks to negotiate resource ownership. Consequently, there are public and private pages stored in global memory. The private pages are those that are modifiable by a user and the public pages are those that are only readable by one or more users.
One aspect of the invention is to manage memory on a computer. From the memory there are a plurality of cache memory blocks cooperatively shared by processing threads executing on the computer. These processing threads include user sessions and resource managers.
The user threads consume page data stored on the cache memory blocks. Each user thread has a public view of unmodified cached pages and can have modified cached pages in a private view. During on-line analytical processing (OLAP), the user threads process the cached pages. For pages that are only read by the user thread, the public view is used to access the necessary cache memory block, which may be read by multiple users. When an analysis requires modifying data, however, access through a public view is inappropriate. Instead, the cache memory block pointed to by the public view is copied to a new cache memory block. The user thread is then assigned a private pointer to the copied pages, and can modify the data in this private view without affecting data viewed by other threads.
The resource managers ensure that the user threads cooperate to function effectively. In particular, a paging manager interfaces the user threads with the cache memory space to retrieve pages from disk.
In accordance with one embodiment of the invention, a computer-implemented program manages memory in a computer having a plurality of memory blocks. These memory blocks can be a cache memory area. Data is stored in memory blocks, including a first memory block and a second memory block. First and second user sessions or user threads execute in the computer, with the first user session having a global view of the first memory block data and the second user session having a global view of the first memory block data and a private view of the second memory block data. In a particular, the first and second user sessions are threads in a multi-threaded computer system.
The user threads can execute resource manager instructions to map data stored in a cache memory block with a location of the cache memory block in the computer. The resource manager also transfers data from a database into a cache memory block and stores generational views of the data. In particular, the data is retrieved from a multi-dimensional database.
A particular method facilitates simultaneous analysis of data in multiple sessions in a computer. First, data is retrieved from storage into public blocks of a shared memory space. These public blocks store data for global read access by a plurality of user sessions. Second, public blocks of data are selectively copied into private blocks of the shared memory space. Each private block stores data for private read and write access by a single user session. Upon read access to a data item by a user session, the data item is read if present from a private biock accessible by the user session. If the data item is not present on a private block accessible by the user session, the data item is read from a public block. Upon write access to a data item by the user session, the data item is written to a private block if present in a private block accessible by the user session. If the private block is not already present, then data is copied from a public to a private block for access by the user session.
A dedicated collector task can be used to reclaim memory blocks. A list of free memory blocks is stored in the computer. A triggering event is generated based on the amount of free memory blocks in the free list. The triggering event triggers a dedicated collector to reclaim memory blocks to the free list.
The user sessions and the dedicated collector task can be implemented as processing threads in a multi-threaded computing system. In particular, the computing system can include a plurality of processing units for executing the threads. However, aspects of the invention can also be applied to processes in a multi-process architecture. As such, the term data accessor will be understood to encompass any computing mechanism to access or manipulate data, including threads and processes. By utilizing a dedicated collector thread, any processing penalty encountered during the reclamation process is absorbed by the collector thread. Thus the user session threads continue to operate normally, making the reclaiming of cache memory blocks by the dedicated collector task thread transparent to the user session threads.
As the number of user session threads concurrently executing in the system increases however, the number of allocatable cache memory blocks stored on the free list decreases. The use of a single dedicated collector task thread can reduce performance of the system because, after requesting a memory block, if the free list is empty, a user session thread must wait until the single dedicated collector task thread reclaims a memory block and stores it on the free list. Also, deadlock can occur if there are no memory blocks on the free list. For example, the single dedicated collector task thread could be waiting for a user session thread to complete after calling the user session thread to perform an action for the single dedicated collector task thread and the user session thread could be waiting for the single dedicated collector task thread to add a memory block to the free list. There is consequently a need for an alternative method for collecting memory blocks for use by multiple user session threads in a multi-threaded computer system.
In particular, the user thread collector can be a routine executed in the user thread. The collector searches shared memory for a collectable block by, for example, randomly selecting an index to a block in shared memory and determining if the selected block is collectable. Upon determining that a previously selected block is not collectable, the user thread collector can successively select a next index to search and determine if the selected next block is collectable. The next index may be selected by incrementing the previously selected index. The actions of selecting and determining can be repeated until a collectable block is found.
By allowing each user thread collector to request reclamation of a block for use by the user thread, the user thread does not have to wait for a single dedicated collector thread to reclaim a block. Thus, potential deadlock is avoided. Also, with each user thread collector in a user thread responsible for reclamation of blocks, by randomly selecting an index to a block in shared memory, avoids potential deadlock and the need for a free list of blocks and the associated logic for controlling the free list is no longer required, reducing the memory required in the computer system.